Graphite materials have excellent properties such as electrical conductivity, lubricity, corrosion resistance and heat resistance, and are being used in wide applications requiring electrically conductive, heat-resistant and/or corrosion-resistant materials. In these applications, graphite materials are usually used in the form of molded articles consisting of graphite alone or a combination of graphite and other materials. Among others, graphite powder occupies an important position as a material for the formation of such molded articles and as a solid lubricant.
Graphite powder for use in these applications, especially when incorporated into rubber and synthetic resins to impart thereto functional properties such as electrical conductivity and thermal conductivity, should preferably comprise fine particles having a high aspect ratio because they can be uniformly dispersed so as to have many mutual contacts.
In the prior art, graphite powder has usually been prepared by a wet or dry grinding method for mechanically reducing natural or synthetic graphite to powder. However, these grinding methods for preparing graphite powder involve several problems. Specifically, it is difficult to reduce graphite to fine particles because the crystallinity of graphite is so developed that slip occurs between graphite crystal layer planes. Even if the grinding power is increased or the grinding time is prolonged, fine particle having a uniform shape cannot be obtained and, moreover, the grinding efficiency is reduced. On the other hand, where graphite powder is prepared by grinding expanded graphite obtained, for example, by heat treatment of intercalated graphite, it has been difficult to reduce expanded graphite to fine particles according to common grinding methods using direct mechanical loads or impacts effected by a mortar with a pestle, a grinder or the like. Specifically, the crystal layer planes of expanded graphite tend to become oriented perpendicularly to the direction of the loads or impacts, resulting in the formation of thin films. Moreover, expanded graphite particles are so soft that they are easily squashed and pressed into a plate-like mass. Furthermore, expanded graphite particles having a bulk density of 0.003 to 0.006 g/cm.sup.3 are so bulky and light that they tend to scatter during the grinding process. In order to solve these problems, Japanese Patent Laid-Open No. 127612/'86 (DERWENT abstract 86-194944/30) discloses a method for preparing an electrically conductive graphite material wherein expanded graphite is ground while the interstices thereof are impregnated with a liquid or, in addition, the liquid is frozen. This method can solve the problem of particle scattering, but still involves several problems to be solved. Specifically, since this method is based on grinding by direct mechanical impact forces, it is desirable to impregnate the interstices of expanded graphite completely with a liquid, and additional operation therefor is required. Moreover, this method produces considerable noises and vibrations associated with ordinary mechanical grinding, Furthermore, although the expanded graphite is impregnated with a liquid, it is difficult to grind it uniformly. Thus, the resulting particles are not uniform in shape and excessively ground particles tend to cohere or form lumps.
In short, it has been difficult to prepare fine graphite particles having uniformly distributed small sizes and high aspect ratios, according to prior art grinding methods. Thus, fine graphite particles having such a shape has been unknown in the prior art.